The present invention relates to a method for producing and refining molten metal by introducing the metal-containing material, carbonaceous material, flux and O.sub.2 gas into the molten metal.
The present invention is particularly applied to the following two refining methods. The first application is the smelting reduction method. The smelting reduction method will replace the blast furnace method which has been the primary method of producing iron for at least four hundred years. The blast furnace method has some disadvantages: It requires many appurtenant facilities such as coke oven plant and sinter plant, an expensive material such as coking coal and high quality ore, a high construction cost and a large space. Hence the smelting reduction method was developed recently as a technique which is free of such disadvantages.
The facility of the typical smelting reduction process mainly comprises a smelting reduction furnace and a prereduction furnace. The outline of the smelting reduction process is as follows:
In the prereduction furnace the metal oxide ore is pre-heated and prereduced by the gas discharged from the smelting reduction furnace. After that, the prereduced metal oxide ore is charged into the smelting reduction furnace together with a carbonaceous material such as coal and flux. O.sub.2 gas and a stirring gas are introduced into the smelting reduction furnace, and the carbonaceous material will be dissolved into the molten metal generated in advance in the smelting reduction furnace. At the same time, the carbon in the carbonaceous material will be combusted by the O.sub.2 gas (hereinafter referred to as "main O.sub.2 gas") to produce CO gas and heat. This heat of combustion will be utilized to melt and finally reduce the prereduced metal oxide ore with carbon and to produce molten metal. The CO gas will be subjected to the secondary combustion by O.sub.2 gas being blown into the furnace from a system which is separate from that of the main CO.sub.2 gas (hereinafter referred to as "secondary combustion O.sub.2 gas"). The heat generated by this combustion is also recovered in the molten metal and used in melting and final reduction of the ore.
The second application is the metallic scrap melting method. Just like the above-mentioned smelting reduction method, this process uses the heat generated by the combustion of carbon in carbonaceous material with O.sub.2 gas to melt scrap.
The most important task in the above-mentioned process is to most effectively recover the heat generated by the second combustion in the furnace. About 80% of the heat given to the furnace is carried out in the form of CO gas. To effectively utilize the enormous latent heat in the form of this CO gas, the second combustion method must be applied. The following methods are known as prior art techniques relating to this second combustion.
The Japanese Patent Provisional Publication No. S-62-280311 discloses an invention concerning a "smelting reduction method characterized in that metal in the metal bath 31 (see FIG. 8 herein) generated by smelting reduction is splashed by a gas injected into the furnace to carry the metal into the second combustion zone," as shown in FIG. 8. (Hereinafter referred to as the "prior art I.")
The Japanese Patent Provisional Publication No. S-64-68415 discloses an invention relating to a "method for producing molten stainless steel by smelting reduction characterized in that in a smelting reduction furnace having a bottom-blown tuyere 41, a side-blown tuyere 42 and a top-blown lance 43 (see FIG. 9 herein), Co gas or/and an inert gas is blown into the furnace together with Cr ore through the bottom-blown tuyere 41, CO or/and an inert gas is blown into the furnace through the side-blown tuyere 42 so that at least a part of the gas flow strikes against a molten metal swell (A) due to the bottom-blown gas, main O.sub.2 gas is blown into the molten metal through the top-blown lance 43, and O.sub.2 gas for second combustion is blown into the slag from the side of the top-blown lance 43 to melt and reduce Cr ore, and after that the specified decarbonization process is given" as shown in FIG. 9. (Hereinafter referred to as the "prior art II.")
The Japanese Patent Provisional Publication No. H-1-205016 discloses an invention relating to a "smelting reduction method and its apparatus, said method being characterized in that iron ore is charged together with coal and a flux into a smelting reduction furnace 51 (see FIG. 10 herein), an inert gas and CO or process gas are blown into the furnace through a bottom-blown tuyere 52 and a side-blown tuyere 53, main O.sub.2 gas and second combustion O.sub.2 gas are blown into the furnace through the top-blown lance 54, at least a part of the gas flow from the side-blown tuyere 53 strikes against the molten metal swell (B) due to the gases blown into the furnace through the bottom-blown tuyere 52, and powdery coal or steam is blown into the furnace to control the oxidation degree of the exhaust gas," as shown in FIG. 10. (Hereinafter referred to as the "prior art III.")
Furthermore, the Japanese Patent Provisional Publication No. S-61-221322 discloses an invention relating to a "smelting reduction method characterized in that a large amount of slag 61 (see FIG. 11 herein) is maintained over a metal bath 62, a part of combustible gas generated in the furnace is combusted with oxygen-containing gas and the generated heat is transferred to the slag 61, the slag 61 is stirred or circulated by gas to effectively transfer heat retained in the slag to the metal bath 62 or the metallic material (C)," as shown in FIG. 11. (Hereinafter referred to as the "prior art IV.")
The above-mentioned prior arts I-IV, however, have the following problems.
In the prior art I, the metal in the metal bath 31 is splashed off, by the O.sub.2 gas from a tuyere for splash forming 32, into a second combustion zone above the slag 33. The second combustion is performed by O.sub.2 gas injected from the second combustion tuyeres 34. In this case, since the second combustion is performed above the slag 33, most of the heat generated by the second combustion is carried away by the exhaust gas although a part of the heat of combustion is transferred to the metal. Thus the heat can not be effectively recovered by the metal. Moreover, as the heat of radiation of the second combustion significantly raises the temperature of the refractories, on the inner side wall of the furnace, the refractories will suffer a greater damage.
In the prior art II, the second combustion is performed in the slag 44 by the second combustion O.sub.2 gas blown into the furnace through the top-blown lance 43. The feed rate of the second combustion O.sub.2 gas blown into the furnace, however, is limited. Moreover, even if the slag is strongly stirred by the gas blown into the furnace through the side-blown tuyere 42, it is difficult to achieve the complete mixture and combustion of the second combustion O.sub.2 gas and the gas to be combusted (CO gas). A fairly large amount of CO gas escapes from the molten metal through the slag layer without meeting the second combustion O.sub.2 gas. Under such circumstances, if the feed rate of the second combustion O.sub.2 gas is increased to improve the second combustion rate, a larger part of the O.sub.2 gas will be left unreacted, and this unreacted O.sub.2 gas will burn above the slag 44, and just as the case of the prior art I, the heat of combustion will be carried away by the waste gas and can not be utilized effectively. Moreover, the heat of radiation of the second combustion will give a greater damage to the refractories in the furnace.
In the prior art III, the second combustion is performed in the slag 55 by the second combustion O.sub.2 gas blown into by the top-blown oxygen lance 54, and has disadvantages similar to those of the prior art II.
The prior art IV has an advantage that the second combustion is stably performed since the large quantity of slag bath 61 serves as the buffer in the chemical processes and/or as the heat insulating layer. This prior art IV, however, has disadvantages similar to those of the above-mentioned prior art II or III.
The present invention was made in view of the above-mentioned disadvantage of the prior art, and is intended to provide a molten metal producing and refining method which gives no damage to the apparatus, realizes a stable and high second combustion rate and effectively recover the heat generated by the second combustion.